1. Field of the Invention
This invention relates to a steel alloy for a tube for use in a combustion atmosphere where V, Na, S and Cl are present, e.g. in the combustion atmosphere of a boiler burning heavy oil, tar, coal or the like or of a garbage incinerator, more particularly to a composite steel tube exhibiting hot corrosion resistance and hot erosion resistance in the presence of V.sub.2 O.sub.5, Na.sub.2 SO.sub.4 and NaCl formed in such an environment.
2. Description of the Prior Art
It is well known that the oxides V.sub.2 O.sub.5 and Na.sub.2 SO.sub.4 which form when crude oil, heavy oil or the like is burned in a boiler or other such combustion system build up oxide scale on the systems where they form low-melting-point compounds which induce a type of local corrosive oxidation known as "vanadium attack". The literature in this field points out that alloys of Cr, Ni, Co etc. exhibit a certain amount of resistance to these types of corrosion (see Iron and Steel, Vol. 67, Page 996, for example).
A number of processes have been proposed for production of composite steel tube. In one of these an alloy cladding material is provisionally attached to a carbon or low-alloy steel, the result is subjected to hot rolling to obtain a clad steel plate and the clad steel plate is formed into composite steel tube by submerged arc welding or the like. There have also been proposed methods of directly metal cladding a finished product to obtain a composite steel tube. For example, Japanese Patent Public Disclosure Sho 61-223106 discloses a method of directly producing a final composite steel tube by fixing a high-alloy powder to a metal material by hot isostatic pressing.
In thermal power plants and the like which use the energy released by burning a fossil fuel or garbage, when the fuel is tar, coal, heavy oil or garbage containing plastic materials, the combustion products of the fuel often contain large amounts of V, Na, S and Cl. These elements in turn cause low-melting-point compounds containing V.sub.2 O.sub.5, Na.sub.2 SO.sub.4, NaCl and the like to form on the surfaces of the furnace wall tubes, steam superheating tubes etc. of the thermal power or incineration facility. As a result, the scale formed on the tube surfaces melts and causes hot corrosion. Over long periods of operation this causes breakage of the furnace wall tubes, steam superheating tubes etc.
Moreover, in the case of a dedicated coal-fired boiler or a garbage incinerator/power plant of the fluidized bed furnace type, accelerated hot corrosion of the furnace wall tubes, the steam superheating tubes etc. is caused by the hot erosive effect of the ash or bed sand.